Q30. Conversion of pyruvate into ethanol and carbon dioxide is associated with which process?
[RRB Group D (18 September 2022, Shift 3)]
(a) Diffusion
(b) Translocation
(c) Fermentation
(d) Transpiration
Answer: (C) Fermentation
Explanation: In alcoholic fermentation, pyruvate first loses carbon dioxide to form acetaldehyde, which is then reduced to ethanol. This restores NAD+ so glycolysis can continue under oxygen-limited conditions. Yeasts commonly perform the process, producing bubbles of carbon dioxide in bread dough.
Q29. Where does anaerobic glucose breakdown to lactate take place in human cells?
[RRB Group D (30 September 2022, Shift 2)]
(a) Only in striated muscle tissue
(b) In the cytoplasm
(c) Inside mitochondria
(d) Only inside veins
Answer: (B) In the cytoplasm
Explanation: Glycolysis and conversion of pyruvate into lactate occur in the cytosol, not in mitochondrial compartments. Lactate formation regenerates NAD+, which supports glycolytic ATP production. This pathway is used by red blood cells and may increase markedly in active muscles.
Q28. Which statement about alveoli is INCORRECT?
[RRB Group D (22 August 2022, Shift 3)]
(a) They are surrounded by extensive blood capillaries
(b) Their walls are very thin
(c) They collectively provide a large gas-exchange area
(d) They are finger-like projections of the small intestine
Answer: (D) They are finger-like projections of the small intestine
Explanation: Alveoli are microscopic lung air sacs, not the finger-like intestinal villi. Their thin epithelium, expansive collective surface and close contact with blood capillaries optimise respiratory gas diffusion. Villi instead increase the absorptive surface area of the small intestine.
Q27. During intense running, what happens when muscles receive inadequate oxygen relative to their demand?
[RRB Group D (29 August 2022, Shift 3)]
(a) Pyruvate is converted into ethanol in human muscle
(b) Pyruvate is converted into lactate only when oxygen is abundant
(c) Pyruvate is converted directly into glucose in each muscle cell
(d) Pyruvate can be converted into lactate
Answer: (D) Pyruvate can be converted into lactate
Explanation: Rapid glycolysis in strongly exercising muscle may produce lactate when energy demand exceeds the available oxidative capacity. Lactate formation permits NAD+ regeneration. Although older textbooks link lactate directly to muscle cramps, exercise-associated cramps have more complex causes and are not simply lactate accumulation.
Q26. Which substance is a principal product of alcoholic fermentation besides carbon dioxide and a small ATP yield?
[RRB Group D (11 October 2022, Shift 1)]
(a) Pyruvate
(b) Water
(c) Lactic acid
(d) Ethanol
Answer: (D) Ethanol
Explanation: Yeast fermentation converts glucose-derived pyruvate into ethanol and carbon dioxide after glycolysis. The pathway allows NAD+ recycling rather than adding further ATP beyond glycolysis. Ethanol formation distinguishes alcoholic fermentation from lactic acid fermentation, which predominantly generates lactate.
Q25. Why does keeping fruits in cold storage generally increase their shelf life?
[UPPSC PCS Prelims (2021)]
(a) It completely prevents sunlight from reaching them
(b) It necessarily increases carbon dioxide production
(c) It slows their rate of respiration
(d) It always increases moisture in fruits
Answer: (C) It slows their rate of respiration
Explanation: Low temperatures reduce the rates of enzyme-controlled metabolic reactions, including fruit respiration. Slower respiration consumes stored food reserves less quickly and delays ripening and senescence. Temperature must be controlled carefully, however, because some tropical fruits develop chilling injury.
Q24. Complete the analogy: Tongue : Taste :: Lungs : ______.
[RRB NTPC (3 February 2021)]
(a) Cognition
(b) Circulation
(c) Perspiration
(d) Respiration
Answer: (D) Respiration
Explanation: The tongue contains taste receptors involved in gustation, while lungs are organs of breathing and pulmonary gas exchange. They allow oxygen to enter the bloodstream and carbon dioxide to leave it. Cellular respiration subsequently uses oxygen within tissues to generate ATP.
Q23. Complete the analogy: Kidney : Urine :: Lungs : ______.
[RRB NTPC (13 March 2021)]
(a) Cough
(b) Carbon dioxide
(c) Breathe
(d) Oxygen
Answer: (B) Carbon dioxide
Explanation: Kidneys eliminate nitrogenous wastes in urine, whereas lungs remove carbon dioxide produced by cellular metabolism. Carbon dioxide travels from tissues to lungs mainly in the blood as bicarbonate. Exhalation also removes water vapour, an additional component of expired air.
Q22. The trachea belongs to which human body system?
[RRB NTPC (31 July 2021)]
(a) Excretory system
(b) Circulatory system
(c) Endocrine system
(d) Respiratory system
Answer: (D) Respiratory system
Explanation: The trachea, or windpipe, is a conducting airway connecting the larynx with the two main bronchi. Rings of cartilage keep it patent, while mucus and cilia help trap and remove inhaled particles. It is part of the respiratory system.
Q21. Insects generally take in air through which structures?
[RRB NTPC (4 March 2021)]
(a) Lungs
(b) Gills
(c) Spiracles
(d) Moist skin
Answer: (C) Spiracles
Explanation: Insects possess external respiratory openings called spiracles. These connect to branching air-filled tracheae and tracheoles, which deliver oxygen toward body tissues. The system reduces reliance on blood for oxygen carriage and is distinct from vertebrate lungs and aquatic gills.
